Technical Field
Embodiments of the invention relate to a solar cell, and more particularly to a selective emitter solar cell.
Background Art
Recently, as existing energy sources such as petroleum and coal are expected to be depleted, interests in alternative energy sources for replacing the existing energy sources are increasing. Among the alternative energy sources, solar cells generating electric energy from solar energy have been particularly spotlighted.
A solar cell generally includes a substrate and an emitter layer which are respectively formed of different conductive type semiconductors, for example, p-type and n-type semiconductors. In this instance, the emitter layer is positioned at a light receiving surface of the substrate, and a p-n junction is formed at an interface between the substrate and the emitter layer. A first electrode and a first current collector electrically connected to the emitter layer are positioned on the emitter layer, and a second electrode electrically connected to the substrate is positioned on a surface opposite the light receiving surface of the substrate.
When light is incident on the solar cell having the above-described structure, electrons inside the semiconductors become free electrons (hereinafter referred to as “electrons”) by the photoelectric effect. Further, electrons and holes respectively move to the n-type semiconductor (e.g., the emitter layer) and the p-type semiconductor (e.g., the substrate) based on the principle of the p-n junction. The electrons moving to the emitter layer and the holes moving to the substrate are respectively collected by the first electrode and the first current collector connected to the emitter layer and the second electrodes connected to the substrate.
The efficiency of the solar cell having the above-described structure is affected by a concentration of impurities used to dope the emitter layer.
For example, when the emitter layer is doped with impurities of a low concentration (i.e., when the emitter layer is a lightly doped region), a recombination of electrons and holes is reduced. Hence, a short circuit current density and an open-circuit voltage may increase. However, a reduction in a fill factor is caused because of an increase in a contact resistance.
Further, when the emitter layer is doped with impurities of a high concentration (i.e., when the emitter layer is a heavily doped region), the contact resistance may decrease and the fill factor may increase. However, the short circuit current density and the open-circuit voltage decrease.
Accordingly, a solar cell, for example, a selective emitter solar cell capable of obtaining both advantages of the lightly doped region and advantages of the heavily doped region has been recently developed.
The selective emitter solar cell has the structure in which an emitter layer includes a first emitter portion (i.e., a lightly doped region) and a second emitter portion (i.e., a heavily doped region) and a first electrode and a first current collector are positioned on the second emitter portion. Because the entire area of the emitter layer has a uniform impurity concentration because of the structure of the selective emitter solar cell, the selective emitter solar cell has an efficiency higher than a conventional solar cell.
However, in the selective emitter solar cell, if the first electrode and the first current collector are not formed at a correct location of the second emitter portion, a parallel resistance increases and thus the fill factor decreases. Hence, the efficiency of the selective emitter solar cell cannot be efficiently improved.